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Cu(II) and Ni(II)-1 10-phenanthroline- 5 6-dione-amino acid ternary complexes exhibiting pH-sensitive redox properties.

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APPLIED ORGANOMETALLIC CHEMISTRY
Appl. Organometal. Chem. 2006; 20: 351–356
Published online in Wiley InterScience
(www.interscience.wiley.com) DOI:10.1002/aoc.1064
Materials, Nanoscience and Catalysis
Cu(II) and Ni(II)-1,10-phenanthroline5,6-dione-amino acid ternary complexes
exhibiting pH-sensitive redox properties
Guang-Jun Xu1 , Ying-Ying Kou1 , Li Feng1,2 , Shi-Ping Yan1 *, Dai-Zheng Liao1 ,
Zong-Hui Jiang1 and Peng Cheng1
1
2
Department of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
Tianjin University of Traditional Chinese Medicine, Tianjin 300193, People’s Republic of China
Received 18 January 2006; Revised 26 February 2006; Accepted 6 March 2006
Syntheses, and electrochemical properties of two novel complexes, [Cu(phendio)(L-Phe)(H2 O)](ClO4 )
·H2 O (1) and [Ni(phendio)(Gly)(H2 O)](ClO4 )·H2 O (2) (where phendio = 1,10-phenanthroline-5,6dione, L-Phe = L-phenylalanine, Gly = glycine), are reported. Single-crystal X-ray diffraction results of
(1) suggest that this complex structure belongs to the orthorhombic crystal system. The electrochemical
properties of free phendio and these complexes in phosphate buffer solutions in a pH range between
2 and 9 have been investigated using cyclic voltammetry. The redox potential of these compounds is
strongly dependent on the proton concentration in the range of −0.3–0.4 V vs SCE (saturated calomel
reference electrode). Phendiol reacts by the reduction of the quinone species to the semiquinone
anion followed by reduction to the fully reduced dianion. At pH lower than 4 and higher than 4,
reduction of phendio proceeds via 2e− /3H+ and 2e− /2H+ processes. For complexes (1) and (2), being
modulated by the coordinated amino acid, the reduction of the phendio ligand proceeds via 2e− /2H+
and 2e− /H+ processes, respectively. Copyright  2006 John Wiley & Sons, Ltd.
KEYWORDS: 1,10-phenanthroline-5,6-dione; amino acid; nickel(II); copper(II); cyclic voltammetry
INTRODUCTION
Cytochrome c oxidase (CcO) is the terminal trans-membrane
enzyme of the respiratory electron transport chain of
aerobic organisms; it converts atmospheric oxygen into
water and couples the oxygen reduction reaction to
proton pumping across the membrane.1 – 4 The pumping
of protons results in the creation of an electrochemical
proton gradient which subsequently drives ATP synthesis.5
The proton pumping across the membrane must work
against both electric and pH gradients. The reduction of
O2 to 2H2 O in the catalytic centre of the enzyme is
recognized to provide energy for the process;1 – 4 however,
the exact mechanism of the coupling between the redox
chemistry and the uphill proton translocation is poorly
understood. Much has been learned since the structure
of the enzyme was solved,6,7 but the key residues
involved in redox-coupled proton activity still remain
*Correspondence to: Shi-Ping Yan, Department of Chemistry,
Nankai University, Tianjin 300071, People’s Republic of China.
E-mail: yansp@nankai.edu.cn
Contract/grant sponsor: National Natural Science Foundation of
China; Contract/grant number: 20331020.
Copyright  2006 John Wiley & Sons, Ltd.
unidentified. Although there has been much interest in the
mechanism of proton pumping in cytochrome c oxidase,
there have been few studies on the mechanism of proton
pumping using model compounds. Herein we outline
a proposal for a simple artificial redox-linked proton
pump gate. 1,10-Phenanthroline-5,6-dione (phendio) ligand
could be an interesting candidate for modelling proton
pumping since its quinone moiety is a redox active
species easily reduced to semiquinone and catecholate8
and exhibiting pH-dependent responses. In this context,
we focused on the development of ternary copper(II) and
nickel(II) complexes of 1,10-phenanthroline-5,6-dione with
amino acids [Cu(phendio)(L-Phe)(H2 O)](ClO4 )·H2 O (1) and
[Ni(phendio)(Gly)(H2 O)](ClO4 )·H2 O (2) and investigated
their electrochemical properties. The selection of amino acids
as the second ligand in the complexes may increase the
biocompatibility of these complexes.
EXPERIMENTAL
Material and measurements
L-Phe was purchased from Aldrich. The other reagents were
obtained from Tianjin Guangfu Chemical Reagent Company
352
G. J. Xu et al.
and used without further purification. All the reagents used
were analytical reagent grade. CHN analyses were carried
out on a Perkin-Elemer analyser at the Institute of ElementoOrganic Chemistry, Nankai University.
Infrared spectra were measured on a Bruker Vector 22
FT-IR instrument in the region of 4000–400 cm−1 in KBr
pellets.
UV–vis spectra were measured in the range 190–1400 nm
using a Jasco V-570 spectrophotometer. Samples were
prepared by dissolving the isolated title complexes in
dimethyl sulfoxide, the concentrations being adjusted at 4
or 0.06 mM (1 M = 1 mol dm−3 ) with respect to Cu (II) or
Ni(II).
Electrochemistry
Cyclic voltammetry measurements were performed on
a BAS Epsilon Electrochemical Workstation. All samples
were purged with nitrogen prior to measurements. Sample
solutions of 7 ml volume were mixed in 10 ml vials. A
standard three-electrode system consisting of glassy carbon
working electrode, platinum-wire auxiliary electrode and
a saturated calomel reference electrode (SCE) was used.
The voltage scan rate during the CV measurements was
100 mV/s. Acidic solutions in a pH range between 2
and 4 were adjusted by mixing 0.1 mol dm−3 HClO4 and
0.1 mol dm−3 NaH2 PO4 solutions. The phosphate buffer
solutions in a pH range between 4 and 9 were prepared
using 0.1 mol dm−3 NaH2 PO4 , 0.1 mol dm−3 Na2 HPO4 and
0.1 mol dm−3 Na3 PO4 solutions. The pH measurements were
carried out using a WTW-315i pH meter (WissenchaftlichTechnische Werkstätten Instruments Inc.).
X-ray crystallography
A green single crystal of [Cu(phendio)(L-Phe)(H2 O)](ClO4 )
·H2 O with approximate dimensions of 0.42 × 0.27 × 0.06 mm
was mounted on a glass fibre. Determination of the unit cell
and data collection were performed using MoKa radiation
(λ = 0.71073P) on a Bruker SMART 1000 diffractometer
equipped with a CCD camera. The ω –φ scan technique was
employed.9 Crystal parameters and structure refinements
for the complex are summarized in Table 1. Selected bond
lengths and angles are listed in Table 2. The structure was
solved primarily by direct methods and secondly by Fourier
difference techniques and refined using the full-matrix leastsquares method. The computations were performed with
the SHELXL-97 program.10,11 All non-hydrogen atoms were
refined anisotropically. The hydrogen atoms were set in
calculated positions and refined as riding atoms with a
common fixed isotropic thermal parameter.
General procedure for the synthesis of
complexes (1) and (2)
1,10-Phenanthroline-5,6-dione was prepared according to the
literature method.12 [Cu(phendio)(L-Phe)(H2 O)](ClO4 )·H2 O
(1), a ternary copper(II) complex, was prepared by mixing
a 10 ml methanolic solution of Cu(ClO4 )2 ·6H2 O (185 mg,
Copyright  2006 John Wiley & Sons, Ltd.
Materials, Nanoscience and Catalysis
Table 1. Crystal data and structure
[Cu(phendio)(L-Phe)(H2 O)](ClO4 )·H2 O
Empirical formula
Formula weight
T (K)
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
γ (deg)
3
V (Å )
Z
Calculated density (mg m−3 )
Absorption coefficient (mm−1 )
F(000)
θ range for data
collection(deg)
Limiting indices
Reflections collected/unique
Absorption correction
Data/restraints/parameters
Goodness-of-fit on F2
Final R indices [I > 2σ (I)]
R indices (all data)
Largest difference peak and
hole (e.A−3 )
refinement
for
C21 H20 Cl Cu N3 O10
573.39
293(2)
Orthorhombic
P2(1)2(1)2(1)
7.8114(16)
11.963(2)
25.375(5)
90.000◦
90.000◦
90.000◦
2371.2(8)
4
1.606
1.096
1172
1.88–25.02
−9 ≤ h ≤ 9
−14 ≤ k ≤ 12
−29 ≤ l ≤ 30
13 374/4193
(Rint = 0.0375)
Semi-empirical from
equivalents
4193/6/326
1.066
R1 = 0.0496,
wR2 = 0.1340
R1 = 0.0644,
wR2 = 0.1440
0.939, −0.622
Table 2. Selected bond lengths (Å) and angles (deg)
Cu(1)–O(2)
Cu(1)–N(1)
Cu(1)–N(3)
Cu(1)–N(2)
Cu(1)–O(3)
O(1)–C(9)
O(2)–C(9)
O(4)–C(14)
O(5)–C(15)
N(1)–C(8)
1.935(3)
1.980(5)
1.993(4)
2.019(4)
2.207(4)
1.220(6)
1.281(6)
1.210(6)
1.208(6)
1.481(6)
O(2)–Cu(1)–N(1)
O(2)–Cu(1)–N(3)
N(1)–Cu(1)–N(3)
O(2)–Cu(1)–N(2)
N(1)–Cu(1)–N(2)
N(3)–Cu(1)–N(2)
O(2)–Cu(1)–O(3)
N(1)–Cu(1)–O(3)
N(3)–Cu(1)–O(3)
N(2)–Cu(1)–O(3)
83.72(17)
91.71(15)
166.6(2)
168.32(16)
101.28(17)
81.04(16)
94.98(16)
101.24(18)
91.73(16)
94.39(16)
0.5 mmol), L-Phenylalanine (82.5 mg, 0.5 mmol) and 0.07 ml
(0.5 mmol) triethylamine with heating and stirring under
aerobic conditions for 30 min, 1,10-phenanthroline-5,6-dione
Appl. Organometal. Chem. 2006; 20: 351–356
DOI: 10.1002/aoc
Materials, Nanoscience and Catalysis
(105 mg, 0.5 mmol) in methanolic solution (10 ml) was
added, and then the solution was stirred for about 3 h at
room temperature. Green blocky single crystals suitable for
X-ray diffraction were obtained on slow evaporation of a
methanolic solution of the complex. Yield: 83%. Calculated
for C21 H20 ClCuN3 O10 (%): C 44.0; H 3.52; N 7.33. Found
(%): C 44.6; H 3.50; N 7.28. IR (KBr, cm−1 ): 1704s, 1646s,
1580s, 1489m, 1432s, 1390m, 1355w, 1306m, 1087vs, 852w,
754w, 731m, 706w, 624 m cm−1 (vs, very strong; s, strong; m,
medium; w, weak).
[Ni(phendio)(Gly)(H2 O)](ClO4 )·H2 O (2) was synthesized
by the procedure described for (1) above except that
Ni(ClO4 )2 ·6H2 O and glycine were used in place of
Cu(ClO4 )2 ·6H2 O and L-Phenylalanine, respectively. The
carmine precipitate was filtered off and washed twice with
15 ml methanol and 15 ml water. Yield: 71%. Calculated for
C15 H16 ClN3 NiO10 (%): C 36.58; H 3.27; N 8.53. Found (%):
C 36.47; H 3.40; N 8.48. IR (KBr, cm−1 ): 1700s, 1622s, 1577s,
1480m, 1429s, 1384m, 1302m, 1092vs, 816w, 732m, 710w,
625 m cm−1 (vs, very strong; s, strong; m, medium; w, weak).
Caution! Perchlorate salts of metal complexes containing
organic ligands are potentially explosive. Only small quantity
of material should be prepared and handled with suitable
safety measures.
RESULTS AND DISCUSSION
The metal complexes were characterized by IR, UV–visible
and x-ray structural analyses. The electrochemical properties
of free phendio and the title complexes in aqueous solutions
at various pH values were investigated.
Spectroscopic studies
The IR spectra of these complexes show asymmetric
νas (COO− ) and symmetric stretching vibrations νs (COO− )
of 1646 and 1390 cm−1 ,13 respectively. The difference
between νas (COO− ) and νs (COO− ) stretching frequencies is
greater than 200 cm−1 , which indicates that the carboxylate
groups are coordinated to the metal ion in a monodenate
Cu(II) and Ni(II)-1,10-phenanthroline-5,6-dione-amino ternary complexes
fashion.14 A strong band at 1704 cm−1 , is assigned to
νs (C = O) of the 1,10-phenanthroline-5,6-dione. The very
strong bands at 1087 cm−1 have been assigned to ν(Cl–O)
of perchlorate anions. The electronic absorption spectrum of
[Cu(phendio)(L-Phe)(H2 O)](ClO4 )·H2 O in dimethyl sulfoxide
solution presents two absorption bands, in which the
intense band at 320 nm (ε = 5.25 × 104 dm3 mol−1 cm−1 ) can
be attributed to the π –π ∗ transitions of the coordinated
phendio ligand, and the broad and weak absorption band
at 617 nm (ε = 183.5 dm3 mol−1 cm−1 ) to the d–d transition
of the central Cu2+ ion. The spectra of the complex
[Ni(phendio)(Gly)(H2 O)](ClO4 )·H2 O exhibits a strong peak
at about 305 nm (ε = 2.65 × 104 dm3 mol−1 cm−1 ), which is
assignable to π –π ∗ transitions in the phendio chromophore
and a broad band near 730 nm (ε = 222 dm3 mol−1 cm−1 ) to a
d–d transition of Ni(II).
X-ray structural characterization
The perspective view of [Cu(phendio)(L-Phe)(H2 O)](ClO4 )
·H2 O is shown in Fig. 1. The key bond lengths and angles
are summarized in Table 2. In the complex, the copper(II)
ion is coordinated in a distorted square pyramidal geometry
through the carboxylate oxygen O(2) and the amino nitrogen
N(1) atoms of L-Phenylalanine and two N atoms of the 1,10phenanthroline-5,6-dione in the basal plane and a water
molecule in the apical position. The four equatorial donor
atoms are nearly coplanar, while the copper atom lies
above the plane by 0.1978 Å toward the axial oxygen. Bond
angles around copper in the square plane are influenced
by the bite of the ligands, so that the values 81.04(16)◦
for N(3)–Cu(1)–N(2) and 83.72(17)◦ for O(2)–Cu(1)–N(1)
are ‘compressed’, while those for O(2)–Cu(1)–N(3) and
N(1)–Cu(1)–N(2) are 91.71(15) and 101.28(17)◦ , respectively.
The values of copper–oxygen and copper–nitrogen bonds
are in agreement with those in the literature for similar
geometries.14 As is usual, the copper–O(3) distance of 2.207(4)
Å is significantly longer than those in the square base; the
angles involving O(3) are in the range 91.73(16)–101.24(18),
showing some distortion. Bond distances and angles in the LPhe ligand are as expected and similar to those in previously
reported structures.15
Figure 1. ORTEP diagram of the complex [Cu(phendio)(L-Phe)(H2 O)](ClO4 )·H2 O.
Copyright  2006 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2006; 20: 351–356
DOI: 10.1002/aoc
353
354
G. J. Xu et al.
Materials, Nanoscience and Catalysis
Figure 2. Cyclic voltammograms of phendio in 0.1 mol dm−3
phosphate buffer solutions at several pH values.
Figure 3. Plots of Ec (vs SCE) vs pH for the reduction of
phendio.
Cyclic voltammetric studies
In order to facilitate the electrochemical assignment of the
metal–phendio complexes, the electrochemistry of the free
phendio will be briefly discussed. It should be mentioned that
the cyclic voltammogram for the free ligand was obtained
under the same conditions as that employed in the study
of its complexes. Figure 2 shows the cyclic voltammogram
of phendio in different pH phosphate buffer solutions. The
voltammograms exhibit a couple of redox peaks, but the
peak potential is shifted to the more negative values as
pH increases, and studies at pH > 9 are made difficult
by the base-catalysed decomposition of phendio, which
produces 4,5-diazafluorenone. The broad anodic and cathodic
wave at low pH value are interpreted as the overlapping
of the two closely successive (quinone/semiquinone) and
(semiquinone/catecholate) redox couples [equations (1) and
(2)].8 This assumption, supported by their peak potentials,
was irrespective of the scan rates. It should be noticed
that the Ep of ∼42 mV was obtained at a high pH value,
which is attributed to reduction of the quinone species to
the semiquinone anion followed by reduction to the fully
reduced dianion at the same potential.16
Figure 3 shows the plot of Ec values of free phendio against
pH. There is a breakpoint at ca. pH 4, and the slopes of the plot
below pH 4 and above pH 4 were determined graphically as
ca. −83 and −54 mV/pH, respectively. The slopes in the plot
indicate that the reduction of the phendio occurs via 2e− /3H+
and 2e− /2H+ processes.17 In accordance with this, quinone
molecules undergo two one-electron reductions in H2 O to
give catecholate analogues and simultaneously combine one
or two protons [equations (1) and (2)]. In the range of pH
2–4, the pyridine nitrogen site of phendio was protonated
[equation (3)].18
(3)
The electrochemical properties of [Cu(phendio)(L-Phe)
(H2 O)](ClO4 ) · H2 O (1) and [Ni(phendio)(Gly)(H2 O)](ClO4 )
(1)
(2)
Copyright  2006 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2006; 20: 351–356
DOI: 10.1002/aoc
Materials, Nanoscience and Catalysis
Cu(II) and Ni(II)-1,10-phenanthroline-5,6-dione-amino ternary complexes
Figure 5.
Plots of Ec (vs SCE) vs pH for the
reduction of (A) ([Cu(phendio)(L-Phe)(H2 O)](ClO4 )·H2 O and
(B) [Ni(phendio)(Gly)(H2 O)](ClO4 )·H2 O.
Figure 4. Cyclic voltammograms of (A) ([Cu(phendio)(L-Phe)
(H2 O)](ClO4 )·H2 O and (B) [Ni(phendio)(Gly)(H2 O)](ClO4 )·H2 O in
0.1 mol dm−3 phosphate buffer solutions at several pH values.
·H2 O (2) at a glassy carbon electrode were investigated
using cyclic voltammetry in aqueous solutions having pH
values between 2 and 9. As shown in Fig. 4(A), the cyclic
voltammogram of (1) at pH 2.2 exhibits a pair of waves with
Ea = 0 V (III ) and Ec = −0.13 V (III) vs SCE; Ep = 130 mV
(where Ea and Ec are the anodic and cathodic peak potentials,
and Ep is the potential separation between the peaks),
attributed to redox processes centred on the metal (CuII /CuI ).
An increase in pH promotes the disappearance of this pair
of waves. Analogous to the phendio system, a pair of waves
with Ea = 0.25 V (I ) and Ec = 0.18 V (I), Ep = 70 mV can
be assigned to the two one-electron redox of coordinated
phendio. The electrochemical response is sensitive to the pH,
Copyright  2006 John Wiley & Sons, Ltd.
and the waves shift to more negative potentials as the pH is
increased. If the pH is raised to 7.01, cathodic peak I moves
to −0.03 V and becomes irreversible. In addition, an anodic
wave appears at 0.2 V (II), which can be assigned to the metalcentred oxidation. In acidic medium, there is an overlap
of anodic peaks of CuII/III and phendio ligand. The cyclic
voltammograms of [Ni(phendio)(Gly)(H2 O)](ClO4 )·H2 O (2)
[see Fig. 4(B)] show only one redox couple assigned to
the ligand-centred reduction/oxidation processes and an
increase of pH is associated with a decrease in the wave
couple potentials.
There is one breakpoint at ca. pH 4 in the plot of Ec
of the ligand-centred redox potentials of 1 against pH
[Fig. 5(A)]. The slopes of the plot of Ec of [Cu(phendio)(LPhe)(H2 O)](ClO4 )·H2 O at pH lower than 4 and higher than
pH 4 were roughly −52 and −34 mV/pH, respectively, which
indicate that the reduction of the phendio ligand proceeded
via the 2e− /2H+ and 2e− /H+ processes. The electrochemical
behaviour of [Ni(phendio)(Gly)(H2 O)](ClO4 )·H2 O (2) [see
Appl. Organometal. Chem. 2006; 20: 351–356
DOI: 10.1002/aoc
355
356
G. J. Xu et al.
Fig. 5(B)] is similar to that of complex 1. By comparing the
slopes of complexes 1 and 2 with those of phendio, the
distinct behaviour of the complexes vs the free ligand is
apparent. The most plausible explanation for this obvious
difference concerns the influence of the coordinated amino
acid in that the amido group can combine protons. In
accordance with this, the proton dissociation constants for
the NH2 group of glycine and phenylalanine (pKb,Gly = 9.58,
pKb,Phe = 9.09)19 are larger than that of coordinated phendiol
(6,6, phendiol = 1,10-phenanthroline-5,6-diol).20 In addition,
the number of protons integrated with coordinated phendio
is dependent on pH.
CONCLUSION
Two novel proton pump model complexes, [Cu(phendio)(LPhe)(H2 O)](ClO4 )·H2 O (1) and [Ni(phendio)(Gly)(H2 O)]
(ClO4 )·H2 O (2), have been synthesized and characterized. The
crystal structure of [Cu(phendio)(L-Phe)(H2 O)](ClO4 )·H2 O
shows that the copper(II) ion is coordinated in a distorted
square pyramidal geometry. The electrochemical properties
of these complexes are pH-dependent and the reduction of
coordinated phendio proceeded via the 2e− /2H+ (pH < 4)
and 2e− /H+ (pH > 4) processes, being modulated by the
amino acid ligands through combining protons on the NH2
groups. This behaviour could be an advantage for research
into the mechanism of proton pumps.
Acknowledgements
This work was supported financially by the National Science
Foundation of China (grant no. 20331020).
Copyright  2006 John Wiley & Sons, Ltd.
Materials, Nanoscience and Catalysis
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Appl. Organometal. Chem. 2006; 20: 351–356
DOI: 10.1002/aoc
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